1. Field of the Invention
The present invention relates generally to television tuning systems and more particularly to such systems which operate to "seek" broadcast television signals, as distinct from normal television tuning systems which are tuned in response to a viewer channel selection in the form of an identifying channel number.
2. Description of the Prior Art
A problem which is indigenous to all-channel television tuning systems is that the FCC television channel frequencies are allocated in four disconnected frequency bands, whereas the corresponding channel identification numbers (exclusively used by viewers for identification) run serially, without regard to the frequency band interruptions. In any given locale the allocated television channels may be widely dispersed throughout the VHF and UHF frequency bands, thus burdening the viewer with becoming familiar with an irregular sequence of non-related channel numbers for ordinary viewing.
Mechanical and electromechanical television tuning systems have been in use for many years. They are, however, slow-acting, tend to be noisy and are best suited to limited-channel systems, as distinct from all-channel systems. The development of varactor tuners, which are controllable by simple application of a DC voltage, has eliminated these drawbacks and made possible silent, high speed all-channel tuning. Varactor tuning systems still require fine tuning to compensate for design tolerances, component aging, signal transmission, etc. and, even in limited-channel applications, the receiver must be "set-up" for the particular channels desired.
As fully detailed in the above mentioned Tanaka application many digitally controlled tuning systems have been proposed for eliminating set-up problems and in general, simplifying tuning for the viewer. In summary, there are two principal types of proposed all-channel digital tuning systems employing varactor tuners: an indirect type which uses error signal information to drive the tuner until proper tuning is achieved; and a direct type which applies a selected voltage to tune the tuner independently of a received signal.
Examples of the indirect type of digital system include one described by Doyle and Mills in BTR, Volume 15, Number 2, July, 1969, in which the detection methods are a frequency synthesizer and a marker (or birdy) synchronizing synthesizer. With either detection method, a comparator compares the incoming signal frequency with the input channel number information and, at equality, operates to send a stop signal to a tuner ramp voltage drive.
A paper by Dexter appearing in the June 18, 1970, issue of ELECTRONIC DESIGN describes an indirect type, which is independent of the received signal, in which an electronic counter is employed to measure the local oscillator frequency, by counting with respect to a known time base, and a comparator is used for comparing the last digit in the counter with a preselected number. At equality, a signal is produced to terminate adjustment of the local oscillator frequency. Thus, fine tuning is related to the allocated frequency of the television broadcast station and achieved without reference to the incoming signal.
Examples of the direct type, (but limited-channel) tuning system include that described in U.S. Pat. No. 3,654,557, issued Apr. 7, 1972, which uses an arbitrarily selected binary encoding of channel numbers to designate and address a limited number of invidual potentiometers for supplying corresponding preset voltages to a varactor tuner. Each channel position has a unique binary encoded number which determines the location of its preset potentiometer for coupling to the varactor tuner. A paper by Sakamoto and Ichinohe in BTR, Volume 18, Number 3, August, 1972, discusses an improvement using an eight bit binary encoded signal stored in a shift register which, through a series of logic gates, activates the potentiometer corresponding to the selected channel number. As the author suggests, an all-channel system may be obtained by adding a separate potentiometer for each channel.
There have also been all-channel direct type binary systems proposed which incorporate PROM's (Programmable Read-Only Memories) to develop appropriate tuning voltages for each channel in the tuning spectrum.
The above-mentioned patent, U.S. Pat. No. 3,851,254, issued Nov. 26, 1974 to Merrill and Tanaka, is directed to a channel number computer and discloses method and apparatus for deriving the channel number of a broadcast television signal by sampling the local oscillator frequency with a counting system. The output of a logic gate, in which a reference clock signal gates a local oscillator signal, is used to drive a modular scaler, a units counter and a tens counter to decode the local oscillator output signal into its corresponding channel number.
All of the proposed tuning systems above had various deficiencies which were solved by the system disclosed in the above-mentioned Tanaka application. That application discloses and claims method and apparatus for a television tuning system employing a comparator for comparing an asynchronously counted local oscillator frequency (expressed in terms of channel numbers and modular residue corresponding to intrachannel fractions) with encoded input channel number information. A voltage ramp drives the tuner until a condition of equality exists between the derived and desired channel numbers and the modular residue is within predetermined limits. Thus the tuning system responds to a two digit channel number input to tune to a restricted frequency range or "window" about the frequency corresponding to the designated channel number. With this system fine tuning is not required, and programming or set-up of the receiver is unnecessary.
Many signal seeking tuning systems have been proposed in the past, primarily to avoid the channel number problem and the inconvenience of tuning through non-operating channels or channel positions. No such system has, however, been commercialized, due in part to many of the same problems solved by the Tanaka application system. In many of the proposed systems a ramp voltage generator, under control of a signal detector, supplies a voltage controlled oscillator for scanning of the appropriate frequencies. The received signal is usually filtered and a frequency discriminator is employed to produce a control potential (error signal) whose magnitude and polarity are the analog of the detected difference between the proper IF frequency and the actual signal frequency translated by the IF portion of the receiver. The error signal is used to correct the voltage of the ramp to adjust the tuner in the proper direction to "lock" to the signal. A threshold circuit may be used to skip over signals which are too weak for satisfactory viewing. These systems are confronted with the conflicting requirements of a "capture" range broad enough to pick up signals during sweeping and narrow enough to lock to proper signals only. This imposes significant limits on the sweeping speed.
In addition, in a signal seeking system the combined effects of capture range, lock range and the heterodyne process make it possible for an unwanted signal to "beat" with the local oscillator signal to produce a signal in the IF amplifier of proper frequency. These spurious or unwanted signals may be in the form of noise, image frequencies, or adjacent channel carriers. Many elaborate detection arrangements have been proposed to cope with spurious signals in signal seeking tuning systems. Most require responses to more than one signal characteristic and there are some in the patent literature which detect as many as four distinct signal characteristics in an effort to preclude recognition of an erroneous signal.
Indirect digital type tuning systems are capable of great tuning accuracy, since by employing additional counters, the tuning frequency range may be narrowed to any degree desired. Unfortunately, extremely accurate tuning systems are not commercially acceptable and may, in many cases, be totally inoperative over the range of broadcast signals available. Such highly accurate systems are wholly unacceptable for signal seeking systems of even moderate ramp speed because a fairly broad capture range is desired to avoid missing a legitimate signal. Practically speaking, because of this broad capture range, and the difficulty in distinguishing between desired and undesired signals, signal seeking systems have not been feasible.